1. Field of the Invention
The present invention relates in general to catalytic generation of .alpha.,.beta.-unsaturated carboxylic acids and esters, and more particularly to preparing .alpha.,.beta.-unsaturated carboxylic acids and esters from carboxylic acids and esters using a niobium catalyst.
2. Description of the Related Art
.alpha.,.beta.-Unsaturated acids, particularly acrylic acid (AA), methacrylic acid (MAA), and their esters, are useful organic compounds whose polymeric forms find a myriad of applications, including plastic sheeting for signs, coatings (including latex paints), adhesives, fibers, and synthetic resins. Currently, MAA and methyl methacrylate (MMA) are manufactured industrially primarily by the acetone cyanohydrin process, which uses toxic hydrogen cyanide and generates large quantities of ammonium bisulfate wastes, which have to be disposed or regenerated at substantial cost. Acrylic acid is generally manufactured by a two step oxidation of propylene.
For nearly a quarter of a century, there has been a desire in the industry to replace the current acetone cyanohydrin process for methacrylic acid with a process that eliminates both the environmentally deleterious ammonium bisulfate waste and the safety hazards associated with handling very toxic hydrogen cyanide. Further, both the acrylic acid and methacrylic acid processes are currently dependent upon petrochemical feedstocks. Over the long term, there is a desire to diversify the chemical industry's feedstock options, particularly using synthesis gas, which can be generated from more abundant natural gas, coal, and heavy oil based carbon sources. One process, which has been envisioned as a means for attaining these goals, is shown generically in reaction [1] below. EQU R.sup.1 CH.sub.2 CO.sub.2 H+R.sup.2 CHO&lt;--&gt;R.sup.2 HC.dbd.C(R.sup.1)CO.sub.2 H+H.sub.2 O [1]
In the condensation of formaldehyde with acetic acid to generate acrylic acid, R.sup.1 and R.sup.2 are hydrogen atoms. In the condensation of formaldehyde with propionic acid to generate methacrylic acid, R is a methyl group and R.sup.2 is a hydrogen atom. To understand reaction [1] and the subsequent reactions, it is important to note that the carbon atom (R.sup.2 CHO) shown in the reactant aldehyde forms a double bond with the .alpha.-carbon in the reactant acid. The .alpha.-carbon in the reactant acid is the carbon atom (R.sup.1 CH.sub.2 CO.sub.2 H) adjacent to the acid carbon. R.sup.1 is bonded to the .alpha.-carbon in both the reactant acid and the product acid. However, due to the spatial relationship of the product, R.sup.1 is shown in the middle of the product, whereas R.sup.2 is shown to the left.
One drawback to these prior processes has been the need to generate or import difficult to handle formaldehyde. It would be advantageous if the formaldehyde could be replaced with methanol, which is more easily handled. Unfortunately, when methanol has been successfully used to form carbon-carbon bonds, the products are generally simple alkylation products via reaction [2]. The more desired reaction, is reaction [3]: EQU R.sup.3 CH.sub.2 CO.sub.2 R.sup.4 +R.sup.5 CH.sub.2 OH&lt;--&gt;R.sup.5 CH.sub.2 (R.sup.3)CO.sub.2 R.sup.4 +H.sub.2 O [2] EQU R.sup.6 CH.sub.2 CO.sub.2 R.sup.7 +R.sup.8 CH.sub.2 OH+O.sub.2 &lt;--&gt;R.sup.8 HC.dbd.C(R.sup.6)CO.sub.2 R.sup.7 +2H.sub.2 O [3]
where R.sup.3 through R.sup.8 are independently a hydrogen atom, a methyl or other alkyl group, or another organic.
Reaction [3] has been accomplished in only very few select cases, entailing the use of a mixture of methanol and oxygen in the presence of catalysts consisting of vanadium and phosphorous. That is, vanadium-phosphorous oxides are used to catalyze the reaction. The use of a phosphorous component is neither simple nor desired. Furthermore, vanadium oxide (V.sub.2 O.sub.5), although extensively used in the industry as a catalyst, is a toxic substance with a myriad of deleterious effects. As such, it is strictly controlled in the environment and work place.
The inventors of the present application developed a condensation reaction of carboxylic acid derivatives with formaldehyde over supported niobium catalysts. However, this development does not contemplate the use of methanol and oxygen as an alternative to formaldehyde. Further, there is no prior art indicating that niobium will effect an oxidation of methanol to formaldehyde, which is an intrinsic requirement for this reaction to proceed.